Unified the Exception classes used by gleam and mp.  The new Exception hierarchy leverages Boost.Exception.

Also there were a few places (mainly gleam/file_io.cpp) where I removed some Win32 specific code which is only
getting in the way at this point since I decided to put off Windows support indefinitely.

When I eventually decide to bring on Windows support, I'll go ahead and reimplement it.  Most of the code deleted
was in gleam/file_io.cpp, which is going the way of the dinosaur eventually anyways...